FIG. 1.

Stage-specific expression of α- and β-like globin genes. Changes in globin gene expression during development are associated with changes in the site of production (yolk sac, fetal liver, adult bone marrow) and morphology (size, presence or absence of nucleus) of erythroid cells. The α-like globin genes are ζ (embryonic) and α (fetal, adult); the β-like globin genes are ϵ (embryonic), γ (fetal), and β, δ (adult). Their protein products assemble into heterotetramers to form functional hemoglobin protein.